Cucumber (Cucumis sativus L.) is a popular vegetable crop that has been cultivated for several thousand years and is grown worldwide. Cucumber plants are grown in a wide range of climates, and in open fields as well as greenhouses. The two main types of cucumber fruit grown commercially today are fresh market (slicing) and processing (pickling).
Downy Mildew (DM) is caused by the fungus Pseudoperonospora cubensis (P.c.), which causes significant crop losses among many Cucurbit species, including cucumber. The disease is found worldwide and favors moist, temperate conditions. The disease affects greenhouse grown plants, and plants grown in the field. DM is one of the most important foliar diseases of cucurbits, and can reduce fruit yield and quality, and may kill susceptible seedlings.
Symptoms of DM infection are variable. Initial symptoms include sharp, irregular yellow lesions on the upper surface of the leaves, which eventually become more distinct on both sides of the leaves. The underside of the leaves may exhibit a whitish-gray, brown, or light blue growth, particularly under moist conditions. This downy growth is spores produced on the lower surface of the lesion. A general yellowing of affected leaves typically occurs as the lesions coalesce into one large lesion, eventually causing the leaf to wilt and die. The disease can progress quite rapidly, killing foliage in a matter of a few days and resulting in poor fruit production and quality. Cucumber fruit are not affected directly, but major defoliation exposes the fruit to sunscald. Once it appears on a crop, DM rapidly spreads by wind, or splashing rain and/or irrigation water. Disease management and prevention requires destruction of all plants from infected nurseries and disinfection of the facilities. Emergence of a new isolate of DM has also overcome some previously known resistant lines. Thus, there is a need for new cucumber varieties having resistance to DM.